Generally, BPSK is a method for modulating (0, 1) of data to (0, π) of the phase of a carrier, and is often used in the field of telecommunications. In recent years, this modulation technique has been also used in the field of optical disks, which are a type of recording medium. For example, this modulation scheme is used in DVD+R/+RW.
In the case of optical disks, such as DVD+R/+RW, to which data can be written, it is necessary to accurately recognize the present position of the optical pickup, that is, the present address information. These optical disks thus employ a method in which a recording track formed on the media is wobbled periodically, and in specific positions of the wobbled groove (hereinafter referred to as wobble), phase inversion is caused to occur so as to represent addresses.
When the wobble is read by the optical pickup, an analog signal having the wobble period is obtained. It is a BPSK demodulator that detects a phase from this analog signal and reproduces address information. The above-mentioned technique is disclosed for example in Japanese Laid-Open Publication Nos. 5-260413, 2001-126413, and 2001-209937.
Many conventional BPSK demodulators are composed of analog circuits as shown in FIG. 7. In FIG. 7, the reference numeral 701 denotes a carrier reproduction circuit for reproducing a carrier of a BPSK modulated signal from the BPSK modulated signal. Although not shown, the carrier reproduction circuit is often composed of a bandpass filter (hereinafter referred to as a “BPF”) and a PLL circuit. The reference numeral 702 indicates a BPF for also receiving the BPSK modulated signal and then passing only the frequency band of the signal to remove noise in the transmission path. The reference numeral 703 represents a multiplier for receiving the reproduced carrier from the carrier reproduction circuit 701 and the BPSK modulated signal whose frequency band alone has been allowed to pass by the BPF 702, and then multiplying the two inputs. When the reproduced carrier and the BPF output signal are in phase with each other, an output value produced from the multiplier 703 is always a positive sine wave. When they are in opposite phase, on the other hand, the output value is always a negative sine wave. Only a low frequency band of the multiplication result produced from the multiplier 703 is allowed to pass by a lowpass filter 704 (hereinafter refereed to as a “LPF”). Theoretically, a constant positive value is output from the LPF 704 in the case where the reproduced carrier and the BPF output signal are in phase with each other, while a constant negative value is output in the case where they are in opposite phase. The reference numeral 705 denotes a discriminator for making a comparison between the output produced from the LPF 704 and 0 level. If the value obtained from the comparison is positive, the discriminator 705 outputs 1, and if the value is negative, the discriminator 705 outputs 0. The discrimination result produced from the discriminator 705 is the data before BPSK modulation.